1. Field of the Invention
The present invention relates to an apparatus for correcting an unevenness in an intensity of illumination of an image produced by a copying machine capable of producing variable magnification images.
2. Description of the Prior Art
In an optical slit type exposure copying machine, an unevenness in the intensity of illumination occurs in a longitudinal direction of an optical slit. The unevenness varies in proportion to the fourth power of the cosine of .omega., where .omega. is the half view angle of a lens.
In order to solve the problem created by the nonuniform illumination, conventionally two methods have been used. The first method compensates for the varying light distribution generated by a light source which is used to illuminate an original in such a way as to compensate for the reduced intensity of illumination of an image of the original which passes through opposite longitudinal sections of said longitudinal optical slit. The second method is to vary the width of the optical slit along the longitudinal direction thereof.
However, in a copying machine with a variable magnification, when the magnification is varied, the rate of the unevenness in the intensity of illumination along the longitudinal direction of the optical slit also varies because the half angle of view on the picture surface varies. FIG. 1 is a view which explains this. FIG. 1 shows the distribution of the intensity of illumination on the picture surface using a conventional copying machine illumination means. In FIG. 1, reference numeral 20 denotes a curve which shows an illumination distribution for a reducing magnification. In this case, the illumination intensity is high at the peripheral longitudinal positions of the longitudinal optical slit. Accordingly, when copying, the density of the copy is uneven. That is, the density at the peripheral part of the copy is thin. To the contrary, reference numeral 21 denotes a distribution of an illumination intensity for an enlarging magnification. In this case, the illumination intensity is low at the peripheral longitudinal portions of the longitudinal optical slit. Accordingly, when copying, a fog is created at the peripheral parts of the copy.
To solve these problems, a number of members, one member for each different magnification of the copier, are provided for regulating the luminous flux of light and for partially interrupting the passage of light. These numerous members for regulating the luminous flux of light are chosen and driven in such a way that the width of the optical slit is varied, thereby compensating for the unevenness in the intensity of illumination on the picture surface.
However, the cost for the means to individually drive the plurality of members as well as the cost of the plurality of members themselves is very high. In addition, the optical means is also required to be large.